1. Field of the Invention
The present invention relates to gyroscopes, and more particularly, to a micro-electro-mechanical (MEM) gyroscope fabricated on a piezoelectric substrate.
2. Description of the Prior Art
An emergence of new consumer and automotive products that require angular velocity information has created an increasing demand for a smaller and inexpensive gyroscope or angular rate sensor. Rotating wheel gyroscopes, fiber optic gyroscopes and ring laser gyroscopes have been used extensively for inertial navigation and guidance systems; however, these gyroscopes are too bulky and expensive for newly emerging applications. A smaller and less expensive gyroscope is required for applications such as (i) automotive safety products, e.g., anti-skid system, ABS, airbag system, (ii) consumer products, e.g., 3-D pointer, camcorder, global positioning systems, sports equipment, (iii) industrial products, e.g., robots, machine control, guided vehicles, (iv) medical products, e.g., wheel-chairs, surgical tools, body movement monitoring, and (v) military products, e.g., smart ammunition, new weapon systens. A micri-electro-mechanical (MEM) sensor generally offers advantages of lightweight, small size, low power consumption, and low cost, particulary when manufactured using standard IC-fabrication techniques.
For any mechanical gyroscope, there is a stable reference vibrating motion (V) of a mass (m) such that an angular rotation (Ω) perpendicular to the direction of the vibrating motion (V) causes a Coriolis force perpendicular to the directions of both the vibrating motion (V) and the angular rotation (Ω), at the frequency of the vibrating motion (V). Therefore, the effect of the Coriolis forceF=2·m·V×Ωis a measure of the rate of the angular rotation (Ω).
A mass constrained by a stiffness element in a frame can be placed into an oscillatory motion in a z-direction by an input power source. If the frame is rotated about an x-axis, the oscillatory mass will experience a Coriolis force, in a y-direction, proportional to the applied rate of rotation. The Coriolis force acting on the mass attempts to cause a displacement of the mass in the y-direction proportional to the rate of rotation.
A conventional MEM gyroscope is a silicon-based vibratory sensor that utilizes an energy transfer between two vibrating modes of a mechanical structure. To achieve high sensitivity when subjected to a rotation, the energy from the vibrating modes must be efficiently transferred at a high Q level from an exciting direction to a sensing direction. A considerable effort is required to design and fabricate the vibrating structure and its support electronics to achieve a resolution of less than one degree per second.
Conventional MEM gyroscopes suffer from an inherent performance limitation because of their underlying operating principle, which is based on a vibration of a suspended mechanical structure, i.e., a comb structure, a beam, a disk, or a ring structure. It is often difficult and expensive to design and fabricate a mechanical structure with matching resonant frequencies of the two vibrating modes. The cost of the final product may also increase due to a need for electronic circuitry for controlling and detecting the status of the vibrating structure in order to improve dynamic range. In addition, the suspended vibrating mechanical structure is susceptible to external shock and vibration that occurs at frequencies not far removed from the frequency at which the gyroscope operates. Such disturbances can influence the vibrating structure. Consequently, the structure cannot be rigidly attached to the substrate for its resonant vibration, thereby also limiting its dynamic range.
For example, in a vibratory gyroscope such as a tuning fork gyroscope, the tuning fork consists of two tines connected to a junction bar. In operation, the tines vibrate at a designed frequency. When the tuning fork is subjected to rotation at its sensitive axis, a Coriolis force causes an orthogonal vibration. The Coriolis force can be detected from a differential bending of the tines. The tines and the vibrating structures are susceptible to external shock and vibration that occurs at frequencies close to the tine vibration frequency. Such disturbances can influence the vibrating structure and produce erroneous results. This is a major draw back of a vibrating gyroscope.
U.S. Pat. No. 6,003,370 to Yukawa et al. (hereinafter “the Yukawa et al. patent”), entitled ELASTIC SURFACE WAVE GYROSCOPE, relates to an elastic surface wave gyroscope for detecting a Coriolis force generated on the surface of a piezoelectric substrate by the interaction of a surface oscillation caused by an elastic surface wave of the piezoelectric substrate and a rotary motion of the piezoelectric substrate. Two transducers on a piezoelectric substrate generate two elastic surface waves of different frequencies. Two pairs of reflectors for reflecting the two elastic surface waves produce two different standing waves. The two standing waves interfere with one another and produce an interference wave adapted for detecting the Coriolis force. Another pair of transducers fixed between the first two transducers will detect waves produced by the Coriolis force.
One disadvantage of the gyroscope of the Yukawa et al. patent is its susceptibility to cross coupling between the frequencies of the first two transducers. One technique for reducing cross coupling is to design the transducers to operate in a very narrow frequency band. Narrow band operation can be achieved by using a large number of IDTs, but unfortunately this increases the size of the gyroscope. Another alternative is to design the two transducers to operate with a very large difference in frequency. However, one transducer may be susceptible to harmonics of the frequency of the other transducer. For example if the devices are designed for 40 MHz and 80 MHz, then the 80 MHz device will receive the 40 MHz frequency because its harmonic frequencies are 80 MHz, 120 MHz, 160 MHz . . . and so on.
Another disadvantage of the gyroscope of the Yukawa et al. patent involves the arrangement of the third transducer between the first two. The third transducer can receive harmonic frequencies from the first two transducers and produce a high output signal even without rotating. Also, during operation of the gyroscope, the signal due to rotation will be small and difficult to detect in the presence of the first and the second classic surface waves.